#include <fstream>

int main()
{
	std::ifstream input("in.txt");
	std::ofstream output("out.txt");

	double x_1, y_1;
	double x, y;
	double prev_x, prev_y;
	double area = 0.;

	input >> x_1 >> y_1;
	prev_x = x_1; 
	prev_y = y_1;

	while (input >> x >>y)
	{
		area += (x - prev_x) * (y + prev_y);
		prev_x = x;
		prev_y = y;
	}

	area += (x_1 - prev_x) * (y_1 + prev_y);
	area = abs(area);
	area /= 2;
	output.precision(12);
	output << area;
	return 0;
}